Korean J. Chem. Eng., 26(3), 913-918 (2009)
SHORT COMMUNICATION
Catalytic dry oxidation of aniline, benzene, and pyridine adsorbed on a CuO doped activated carbon Bingzheng Li*,**, Zhenyu Liu*,***,†, Zhiping Lei*,****, and Zhanggen Huang* *State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China **Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China ***State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China ****School of Chemical & Chemistry, Anhui university of Technology, Maanshan, 243002, P. R. China (Received 31 July 2008 • accepted 27 December 2008) Abstract−Adsorption of aniline, benzene and pyridine from water on a copper oxide doped activated carbon (CuO/ AC) at 30 oC and oxidation behavior of the adsorbed pollutants over CuO/AC in a temperature range up to 500 oC are investigated in TG and tubular-reactor/MS systems. Results show that the AC has little activity towards oxidation of the pollutants and CuO is the active oxidation site. Oxidation of aniline occurs at 231-349 oC and yields mainly CO2, H2O and N2. Oxidation of pyridine occurs at a narrower temperature range, 255-309 oC, after a significant amount of desorption starting at 150 oC. Benzene desorbs at temperatures as low as 105 oC and shows no sign of oxidation. The result suggests that adsorption-catalytic dry oxidation is suitable only for the strongly adsorbed pollutants. Oxidation temperatures of CuO/AC for organic pollutants are higher than 200 oC and pollutants desorbing easily at temperatures below 200 oC cannot be treated by the method. Key words: Catalytic Dry Oxidation, CuO Doped Catalyst, Aniline, Benzene, Pyridine, Desorption
INTRODUCTION
zene, are adsorbed and tested through temperature programmed oxidation (TPO) and temperature programmed desorption (TPD) with the aid of mass spectroscopy to understand their oxidation behaviors over CuO/AC.
There are usually many aromatic compounds in wastewater streams from chemical factories [1]. These compounds are toxic and/or carcinogenic [2] and should be effectively removed before discharging of the streams. Many techniques including adsorption [3], aerobic and anaerobic biodegradation [4], ion exchange [5], wet air oxidation [6] as well as supercritical wet oxidation [7] have been developed for such a purpose, but the widely used method is adsorption by activated carbons (ACs) [8]. Among various types of AC-based methods, a method for integrating adsorption of organic pollutants on an AC, followed by catalytic dry oxidation of the organic pollutants upon discharging of water (termed adsorption-catalytic dry oxidation, ACDO in short) was found to be promising especially for wastewaters of low pollutant concentrations [9-13]. The ACDO method has a number of advantages including low energy consumption, repeat use of the sorbent/catalyst, and reduced leaching of catalyst component from the sorbent. Our earlier studies [11,13] showed that a CuO doped activated carbon (CuO/AC) sorbent/catalyst is capable to adsorb a significant amount of phenol from wastewater at room temperature and to oxidize the adsorbed phenol into CO2 and H2O at temperatures below 300 oC. It is of interest, therefore, to see the CuO/AC’s ability to adsorb and oxidize other organic pollutants. In this work, three organic compounds, aniline, pyridine and ben-
EXPERIMENTAL 1. Materials Analytical grade aniline, benzene, and pyridine were used. These compounds were dissolved separately into distilled water to yield aqueous solutions with a concentration of 1,000 mg/L for each pollutant. The AC used was a commercial product from Xinhua Chemical Factory (Taiyuan, China), which was crushed into particles of 4060 mesh (0.3-0.45 mm) and dried in air at 110 oC for 48 h. 2. Preparation of CuO/AC and Pollutant-adsorbed CuO/AC CuO/AC were prepared by pore volume impregnation of the AC with an aqueous copper (II) nitrate solution, followed by aging at room temperature for 2 h, drying in air at 50 oC and at 110 oC for 6 h, respectively, and finally calcining in N2 at 250 oC for 2 h. Cu loading of the prepared CuO/AC is 5 wt%, which was determined by the concentration of the copper (II) nitrate solution used in the impregnation. The pollutant-adsorbed AC and CuO/AC were prepared in batch adsorption experiments in the aqueous solutions with an initial pollutant concentration of 1,000 mg/L. 0.500 g AC or CuO/AC was submerged in a 50 ml aqueous solution in a 50 ml flask, which was continuously agitated by a thermostatic automatic shaker at 30 oC and 150 rpm for 3 days. The pollutant-adsorbed sample was then separated out from the solution by filtration, and then subjected to
To whom correspondence should be addressed. E-mail:
[email protected] ‡ This work was presented at the 7th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26-28, 2008. †
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drying in air at 30 oC to a constant weight. The AC adsorbed with aniline, benzene and pyridine are termed AC-ani, AC-ben and ACpyr, respectively, while the CuO/AC adsorbed with aniline, benzene and pyridine are termed CuO/AC-ani, CuO/AC-ben and CuO/ AC-pyr, respectively. The AC and CuO/AC submerged in distilled water at conditions the same as these pollutant-adsorbed samples were prepared and termed AC-blank and CuO/AC-blank, respectively. The amount of a pollutant adsorbed on a sample was determined from the difference in the pollutant concentration in the aqueous solution before and after the adsorption, which were measured on a UV-Vis spectrometer (Techcomp UV2300, China). The wave length for aniline, benzene and pyridine measurements was 230 nm, 200.4 nm and 252 nm, respectively. The adsorption capacity of a pollutant on AC or CuO/AC (qe) is defined as the amount of a pollutant adsorbed on these solid samples in mg/g and calculated by using (Ci − Ce )V qe = ----------------------M
(1)
where Ci and Ce are initial and equilibrium concentrations of the solution in mg/L, respectively, and V is volume of the solution used in L and M mass of the sample in g. 3. Experimental Temperature-programmed oxidation (TPO) experiments were carried out in a TG system to investigate weight loss behavior of samples. The TG apparatus is an STA 409 PC analyzer (NETZSCH). A sample of about 20 mg was loaded in the TG and was heated from room temperature to 500 oC at a rate of 10 oC/ min under a flow of O2/Ar (5% O2, 12 ml/min). A tubular-reactor/MS system was used for the study under the same conditions as that of the TG system, which contains a tubularquartz reactor of 6 mm in id and 370 mm in length, and a Balzers QMG 422 quadrupolar mass spectrometer. Temperature-programmed desorption (TPD) was also carried out in the system under an Ar stream (99.99%) at the same conditions. Pore volume and Brunauer Emmett Teller (BET) surface area of the AC and CuO/AC were measured through nitrogen adsorption at 77 K by using an ASAP 2000 surface area analyzer (USA). The pore volume was calculated by density-function-theory (DFT). X-ray diffraction (XRD) patterns of the samples were obtained on a Rigaku computer-controlled D/max 2500X using Cu-Kα as the radiation source. The applied current and voltage were 100 mA and 40 kV, respectively. The sample was scanned from 5 to 85 oC at a speed of 0.4o/min.
Fig. 2. Equilibrium adsorption of benzene, aniline and pyridine on AC and CuO/AC at 30 oC with an initial concentration of 1,000 mg/L.
RESULTS AND DISCUSSION 1. Characterization of the AC and CuO/AC Table 1 shows texture parameters of the AC and CuO/AC. Compared to the AC, pore volume and BET surface area of CuO/AC are significantly small, indicating filling of the AC’s pores by CuO. Table 1. BET surface area and texture parameters of adsorbents Samples
SBET m2/g
Vmic ml/g
Vmes ml/g
AC CuO/AC
936 786
0.304 0.249
0.113 0.069
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Fig. 1. XRD patterns of CuO/AC.
Fig. 1 shows XRD pattern of CuO/AC. The presence of CuO diffraction peaks and the low signal to noise ratio indicates the CuO is finely dispersed on the AC. 2. Adsorption of Pollutants Fig. 2 shows amounts of aniline, benzene and pyridine adsorbed on the AC and CuO/AC at equilibrium with an initial concentration of 1,000 mg/L. Apparently, supporting CuO on the AC affects adsorption of these compounds only a little, resulting in small adsorption reductions. The amount of benzene adsorption is the largest,
Catalytic dry oxidation of aniline, benzene, and pyridine adsorbed on a CuO doped activated carbon
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98.2 mg/g corresponding to a 98% removal from the solution, while that of pyridine is the lowest, 73.3 mg/g corresponding to a 73% removal from the solution. 3. Oxidation Behavior 3-1. Oxidation of Aniline Fig. 3 shows TG/DTG-MS curves for TPO of CuO/AC-ani and CuO/AC-blank. It can be seen in Fig. 3(b) that compared to CuO/ AC-blank, CO2 release from CuO/AC-ani starts at about 231 oC, reaches a plateau at 305 oC and then speeds up at 349 oC, indicating an initial oxidation temperature of 231 oC for aniline and an initial
Fig. 5. TG/DTG-MS profiles for TPO of AC-blank (dot) and ACani (solid).
Fig. 3. TG/DTG-MS profiles for TPO of CuO/AC-blank (dot) and CuO/AC-ani (solid).
Fig. 4. MS profiles of aniline evolution for CuO/AC-ani in TPO and TPD.
ignition temperature of 349 oC for AC. DTG profile (Fig. 3(a)) of CuO/AC-ani also starts to deviate from that of CuO/AC-blank at around 231 oC and shows a peak at 305 oC. The weight loss recorded for this peak, in a temperature range of 231-349 oC, is about 10.5 wt%. After subtracting weight loss of CuO/AC-blank in the same temperature range, a net weight loss of about 6.8 wt% for CuO/ACani is obtained, which corresponds to 74.7% aniline adsorbed on the CuO/AC. Fig. 3(c)-(e) shows similar N2 and H2O releases, starting at about 231 oC and peaking at 312 oC, and the absence of aniline, benzene, and phenol releases. TPD of aniline (Fig. 4) further shows that aniline desorbs from CuO/AC at temperatures higher than 220 oC. Clearly, these data indicate that desorption of aniline is inhibited by O2, and the adsorbed aniline can be mainly oxidized into CO2, N2 and H2O at a temperatures window of 231-349 oC. Compared to wet air oxidation of aniline (at 5 MPa, 200 oC) [14], the catalytic oxidation over CuO/AC can be effectively operated at an ambient pressure. To understand the role of CuO, AC-ani and AC-blank were also subjected to TPO; the TG/DTG-MS profiles obtained are shown in Fig. 5. Similar O2 and CO2 release profiles for AC-ani and ACblank in Fig. 5(b) indicate little oxidation for aniline. The DTG profile of AC-ani in Fig. 5(a) starts to deviate from that of AC-blank at around 184 oC and shows a DTG peak at about 264 oC. This trend is the same as that for evolutions of aniline, phenol and benzene in Fig. 5(d). The higher initial oxidation temperature of aniline on CuO/ AC-ani (in Fig. 3) than the desorption temperature of aniline from AC-ani in (Fig. 5) suggests promotion of aniline adsorption strength by CuO. These data along with the small evolutions of H2O and N2 indicate that the AC has a very limited catalytic activity for aniline oxidation. 3-2. Oxidation of Benzene MS profiles of CuO/AC-ben and CuO/AC-blank in TPO experiments are shown in Fig. 6. Compared to CuO/AC-blank, CuO/ACben shows significant benzene desorption in a temperature range of 105-275 oC with a peak at 196 oC. The similar O2, CO2 and H2O curves at temperatures below about 196 oC and the slightly higher O2 consumption and the slightly more CO2 and H2O releases at temperatures higher than 196 oC suggest that only a small amount of Korean J. Chem. Eng.(Vol. 26, No. 3)
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Table 2. Comparison of benzene conversion for oxidation between ACDO and fixed bed flow reactor Benzene vapor Benzene adsorbed on in flow reactor CuO/AC in ACDO Temperature Thermostatic Desorption Retention time Uniform
Temperature programmed
